Pseudomonas aeruginosa is a major opportunistic pathogen, and this study will elucidate its pathogenic mechanisms. The ability of P. aeruginosa to secrete numerous toxic and degradative enzymes plays a major role in its pathogenesis. Among these exoenzymes are LasB protease (elastase) and LasA protease (staphylolytic protease) which act synergistically to degrade elastin and many other clinically relevant proteins. Both LasA and LasB proteases are initially formed as pre-proenzymes with three domains: a signal sequence, a propeptide and the mature sequence. The propeptide domain acts as an intramolecular chaperone that is required for both the enzyme's activity and its extracellular localization. The objective of this proposal is to understand LasA and LasB molecular structures, the mechanisms of their chaperone-assisted folding, pathways of secretion in P. aeruginosa, and clinical relevance. This proposal combines molecular, biochemical and biophysical approaches to compare the processing and secretion of these two important proteases. The investigators will analyze the structures of LasA and LasB exoproteins by genetic and crystallographic methods. They will determine the mechanisms of propeptide (chaperone)-mediated folding and secretion of LasA and LasB proteins. They will determine the mechanism by which proLasA and proLasB interact with the type II secretion apparatus for release to the environment. They will also determine the clinical relevance of LasA and LasB in pathogenesis, and test the efficacy of LasA as a specific anti-staphylococcal therapeutic treatment. This research will result in new information on a significant mechanism of pathogenesis in an important opportunistic organism. It also provides a model system for understanding a major pathway of protein export in gram-negative bacteria.